Interview Question for Intern at Amazon.com:

CORRECTION:
It should be printing t -> val, not t -> left Sorry!

void inorder(tree t)
{
   if(t == NULL)
       return;
   inorder(t -> left);
   cout << t -> val << " ";
   inorder(t -> right);
}

void preorder(tree t)
{
   if(t == NULL)
       return;
   cout << t -> val << " ";
   preorder(t -> left);
   preorder(t -> right);
}

Since I'm already this far here is post order as well:

void postorder(tree t)
{
   if(t == NULL)
       return;
   postorder(t -> left);
   postorder(t -> right);
   cout << t -> val << " ";
}